This invention is concerned with waste treatment and in particular the treatment of solvents used in nuclear fuel reprocessing and uranium ore purification. More particularly, the invention is concerned with the treatment of a solvent comprising an organophosphate ester and a hydrocarbon diluent A typical such solvent is tributyl phosphate and diluents are odourless kerosene (OK), dodecane or a hydrogenated propylene tetramer (HPT). The phosphate ester is typically present in an amount of between 5 and 35% by volume.
EP 342876 discloses a process for the treatment of such a solvent in which the alkyl phosphate is reacted with hydrogen peroxide in aqueous solution added progressively at a temperature that is above ambient temperature and in the presence of a transition metal catalyst which is a chromium compound. The aqueous hydrogen peroxide phase is maintained at a pH which is mildly acidic, neutral or mildly alkaline by the controlled introduction of alkali at a rate that is sufficient to neutralise the acid released by decomposition of the alkyl phosphate, thereby significantly accelerating the rate of decomposition of the alkyl phosphate.
EP 428309 discloses a process Li which the alkyl phosphate is reacted with an aqueous solution of an alkaline metal hydroxide at elevated temperature and a part of the reaction product is then reacted with an aqueous solution of hydrogen peroxide in the presence of an effective amount of a transition metal catalyst.
Ginisty, C. et al (Solvent distillation studies for a purex reprocessing plant.xe2x80x9d Sixth Symposium on Separation Science and Technology for Energy Applications. Volume 25, Numbers 13-15. Editor Bell, J. T. ; Watson J. S. Oak Ridge National Lab., TN (USA) 1990. P. 1941-1952 discloses a solvent distilation system for regeneration of purex solvent. The organic residues are subjected directly to a pyrolysis treatment which is not desirable having regard to the safety requirements involved with the disposal of radioactive materials.
EP 0 342 876 A is concerned with the treatment of alkylphosphate and particularly alkylphosphate located in an organic solvent. A process involving hydrolysis and oxidation is proposed.
Motoi, V. et al (xe2x80x9cSolvent purification by distillation and minimising wastes in a reprocessing plant. Purification du solvant par rectification et minimalisation des dechets pour une usine de retraitement.xe2x80x9d Record 87. Nuclear Fuel Reprocessing and Waste Management Vol. 1. Societe Francaise D""energie Nucleaire. 1987. P. 417-421 OF 492 P. Conference: International Conference on Nuclear Fuel Re. France, XP00060207) discloses a process involving the recover of TBP and diluent and suggests that residual organic materials may be subjected to pyrolysis or encapsulation in cement.
According to the present invention there is provided a process for the treatment of a solvent which has been used in nuclear fuel reprocessing or uranium ore purification and which comprises an organophosphate ester and a hydrocarbon diluent, the process comprising distilling the solvent under reduced pressure to remove substantially all the diluent and a major proportion of the organophosphate ester, converting organophosphate to inorganic phosphate and encapsulating the residual material.
The term encapsulation is used to include vitrification or alternative encapsulation methods such as location within cement.